Focusing Problems

I have been visiting this forum for a while but have never posted, until now. I find the forums very informative and friendly and hope someone can help me with a focusing problem I have with my Canon 7D.

I have achieved some great daytime shots with it but recently encountered a problem with low-light photography. I do a lot of landscape stuff and a couple of nights ago I took some shots in low-light with the help of a tripod and remote control. Shooting in RAW I used an aperture of f13 at 20 seconds, 100 ISO, and the shots looked great on the display but when I printed a couple out they weren't very sharp, infact quite blury. When focusing through the viewfinder, in manual focus, I got it bang on so I'm at a loss as to why the image isn't sharp. It's nothing to do with the printing side of things as I've zoomed in on the image on my camera and it's not pin sharp, just like the print. It's obvious I'm doing something wrong at the point of capture but I'm scratching my head in confusion. My previous Nikon D80 images in low-light were fine when using similar techniques and equipment, but my expensive 7D just isn't delivering - or maybe it's me??? I don't use mirror lock-up, could this have sometyhing to do with it?

Here is one of the images in question. It has been downsized, it looks ok until it's zoomed in on the camera display.